The use of gold on electrical contacts is well established in the electronic industry. Gold's high reliability under repeated use, its resistance to corrosion, and low contact resistance, makes it an outstanding material for coating electrical contacts, especially those used in low voltage devices.
Gold is traditionally applied to electrical contacts by electroplating the gold from aqueous solutions of gold complexes, usually cyanides or chlorides. The electronics industry in response to escalating gold prices and ever increasing economic pressures has developed sophisticated equipment for continuous and selective plating of gold in spots and stripes on strips of metal components. There are, however, a number of problems associated with electroplating gold, such as contamination of the baths accompanied by the codeposition of undesirable materials on the contacts; restriction of the range of current usable to obtain optimum plating thus limiting the speed at which components may be plated; waste due to excessive coverage; and hazards associated with the use of such poisonous compounds as potassium cyanide. Concomitant with these are the associated problems of the disposal of the hazardous industrial waste.
The invention disclosed herein, was developed to alleviate a number of the aforementioned problems. This alleviation is primarily achieved by reducing the amount of electroplating to a minimum. Electroplating is required only to plate gold on the stainless steel cylinder. The gold is then transferred from the cylinder to the strips of electrical contact by solid phase welding.
Solid phase welding of gold has been known since antiquity. Studies of early artifacts reveal that gold boxes were welded by pressure welding the joints with a small hammer. Gold inlaid articles were formed by hammering gold into insized lines in various metals. Gold has also been welded to itself and to other metals by clamping the items together and heating the clamped assembly for a period of time.
Dynamic friction welding has also been used to bond gold to itself and other metals. In dynamic friction welding one component is rotated against another under an axial load to generate frictional heat. Once a sufficient temperature has been reached, rotation is then stopped, the weld is completed, and the two components are bonded to each other. The disclosed invention utilizes principles of dynamic friction welding to transfer gold that has been electroplated on a steel cylinder to strips of electrical contacts.
The gold plated cylinder is rotated at a high speed. A strip of electrical contact terminals is held against a rotating cylinder under sufficient normal force to generate frictional heat. As the cylinder continues to rotate, sliding friction is replaced by massive seizure. The gold from the cylinder is welded to the terminal accompanied by the simultaneous shearing of a layer of gold from the rotating cylinder.
The electrical contact terminal to be coated with gold, according to the invention, must be pre-treated in the same manner as electrical contacts which are to be electroplated. A substratum, usually nickel, is first plated on the contact zone in order to prevent diffusion of the base metal into the gold layer. In addition, to accomplish the desired transfer of gold to the contact zone by dynamic friction welding, the substratum must have a shear strength greater than that of the shear strength of pure gold. Nickel also fulfills this requirement. Further, it is important that the electrical contact used with this invention, like contacts that are to be electroplated, be free of organics and other contaminates prior to the application of the gold.